Buck-boost converters are gaining in popularity. Recent applications include portable media players featuring hard disk drives that require a supply voltage of typically 3.3V where output voltages of the batteries most commonly used in portable devices are ranging from 4.2V down to below 3.0V. To be able to make the most of the batteries it is apparent that a voltage regulator is needed that can generate a supply voltage for such components that can either be below, above or equal to the battery voltage. A buck-boost converter does just that.
Buck-boost converters have been known for a long time. A simple non-inverting type converter consists of two switches, two diodes, some sort of modulator to control the switches, an inductor and an output capacitor (see FIG. 1). In the traditional switching scheme the two switches SW1, SW2 are opened and closed at the same time under control of a switch controller 10. By pulse-width modulation, that is by changing the duty cycle of this switching scheme, the output voltage Vout can be held constant with input voltages Vin that are above, below or equal to the output voltage. In other words, a buck-boost converter can act either as a buck converter (input voltage above output voltage), a boost converter (input voltage below output voltage) or as a combination of both according to the control signals applied to it from the switch controller 10. The combination of both types of control is generally applied when the input voltage is approximately equal to the desired output voltage.
Current-mode converters can exhibit sub-harmonic oscillation in the current domain, and slope compensation is generally employed to combat that phenomenon. This involves adding an extra compensating slope to the signal representing the current flowing in the inductor. Any sub-harmonic oscillation is thereby suppressed quickly. Peak current control and valley current control are control modes which may be used.
To understand how the converter of FIG. 1 works let's look at the case where the input voltage Vin is the same as the target output voltage Vout. Let's assume further that this converter works at a constant switching frequency and that the switches and diodes D1, D2 have no resistance. If we now assume that the two switches SW1, SW2 are closed for a time of duration t1 the current in the inductor will flow through the switches and rise according todl/dt1=Vin/L, where L is the inductance of the converter. In the remaining portion of a clock period the switches will be open for t2 and the current in the inductor will continue to flow through the two diodes D1 and D2 and decrease according todl/dt2=−Vout/L. 
In steady-state operation (stable operating point) the current at the beginning and the end of a clock period needs to be the same. If Vin=Vout that means that t1=t2 (see FIG. 3), or in other words a duty cycle D of 0.5.
It can be seen that in this switching scheme current will only be delivered to the output during, in this example, half of the clock cycle, namely when the two switches SW1, SW2 are open. This means, however, that if the load demands a certain amount of current and the DC output voltage is to be held constant, the converter needs to deliver two times that load current when only delivered to the output for half of the clock cycle. Not only does that reduce the efficiency of the converter considerably, in comparison with a simple buck or boost converter, but it also requires a larger inductor due to the larger peak current. This makes this approach not very attractive in a market where users demand smaller devices that they can use for longer.
Attempts have been made to the address the above issues with buck-boost converters based on both voltage-mode and current-mode topologies.
In particular, there has been proposed a buck-boost converter based on a current-mode topology which uses a mixed peak- and valley-current control method called valley-peak current-mode control. In this scheme the converter needs the ability to measure the inductor current at any time during the complete period. In one circuit configuration, this is achieved by adding a sense resistor in a common current path. This adds cost and reduces efficiency. In another arrangement, two current sense circuits are used to sense the current flowing in either of two of the converter switches (corresponding to S2 and S4 in FIG. 2). This adds to the overall circuit complexity, occupied area, and cost.
Consequently, there is a need for improved current control in a buck-boost converter wherein the inductor current is sensed cheaply and effectively.